The present invention relates to control circuits for swept voltage-controlled oscillators in general, and in particular to a method and circuit for maintaining a constant output frequency range when switching from a linear to a logarithmic sweep, or from a logarithmic to a linear sweep.
Voltage-controlled oscillators produce a repetitive output signal, the frequency of which is proportional to an input voltage. It is well known to sweep a voltage-controlled oscillator (VCO) either linearly or non-linearly over a range of frequencies. One application of such swept VCO's is in electronic test and measurement apparatus for generating linear or logarithmic frequency sweeps between adjustable start and stop frequencies. A typical sweep control circuit may include both conventional linear and logarithmic amplifiers driven by a common ramp voltage waveform, the start and stop voltages of which may be set independently to thereby establish the start and stop frequencies of the associated VCO. However, when switching from linear to logarithmic sweep, or vice versa, the VCO frequency shifts because of the differing output voltages of the linear and logarithmic amplifiers. As a consequence, it has been necessary to readjust the start and stop frequencies each time a new sweep mode is selected.